Experimental Study of Energy Exchange Attending Electron Emission from Carbon Nanotubes

Abstract

Phenomena based on nanoscale transport processes offer new possibilities for direct refrigeration by electron emission between opposing electrodes across a vacuum region. The average energy of emitted electrons depends upon the magnitude and shape of the potential energy barrier in the vacuum region, which is affected by the emission gap, emitter work function (potential barrier height), and emitter tip geometry. Emitted electrons are replaced by other electrons to maintain charge continuity, and the difference in energy between the emitted and replacement electrons produces a heating or cooling effect, known as the Nottingham effect, at the emitter surface. Theoretical studies indicate the possibility of very large (> 100 W/cm²) cooling rates, but experimental confirmation is lacking due to challenging material and experimental requirements. To obtain the results discussed in this paper, the energy exchange attending electron emission from multi-walled carbon nanotube (MWNT) array samples is measured with an uncertainty of approximately 1 μ W. The results are found to depend strongly on the adhesive used to bind the MWNT arrays to the substrate, and this effect is explored by using both silver and carbon paints as the adhesive material. An attempt to determine the effect of the emitter work function by intercalating the MWNT arrays with potassium was unsuccessful. Heating curves as a function of the emission current are presented for various sample groups, and these curves provide insight into the mechanisms involved in the energy exchange associated with field emission from MWNT arrays, including the Nottingham effect and Joule heating.

ACKNOWLEDGMENTS

The authors wish to thank the Cooling Technologies Research Center at Purdue University for assistance in funding this project. We are also grateful to Dr. Ronald G. Reifenberger of the Purdue Nanophysics lab for many helpful discussions related to this study.